The hallmark of malignant melanoma is its poor response to chemo- and radiotherapy. Despite advances in understanding of the biology of this tumor type (Meier et al., Frontiers in Bioscience 1998, 3: d1005-1010), the nature of melanoma's protection against radiation-induced apoptosis remains largely unknown. The ability to resist apoptosis by re-arranging the apoptosis machinery, including Fas, tumor necrosis factor receptor 1 (TNFR1), death receptor 3 (DR-3), TNF-related apoptosis-inducing ligand receptor 1 (TRAIL-R1) and TRAIL-R2 (Askenazi et al., Science 1998; 281:1305-1308; and, Nagata, S., Cell 1997; 88: 355-365), is characteristic of most tumor cells, including melanomas (Peli, J., et al., EMBO J 1999; 18:1824-1831). Altered susceptibility to apoptosis was shown to include suppression of the death receptor or increased expression of inhibitory apoptosis proteins (IAP's) that restrain caspase activity (Deveraux, Q., et al., Genes & Dev. 1999; 13:239-252). Common to late-stage melanoma cells is the expression of a large subset of growth factors, cytokines and their respective receptors, which contribute to autocrine and paracrine regulation of their progression (Moretti, S., et al., Int. J. Cancer 1999; 20:160-168). Among the latter are TNFα-TNFR1 and Fas-FasL, whose interaction elicits either death- or survival-signaling cascades. These cascades are regulated by the signal adaptor TNFR-associated factor2 (TRAF2) and its downstream effectors, i.e., stress activated kinases and their respective transcription factors (Hsu, H., et al., Cell 1996; 84:299-308; Liu, Z. -G., et al., Cell 1996; 87:565-576; Arch, R. H., et al., Gedrich, R. W., et al. and Genes & Dev. 1998; 12:2821-2830).
Key signaling molecules documented to play an important role in the biology of melanoma consists of cell adhesion molecules including cadherins, integrins, MUC 18, ICAM (Johnson, J., Cancer Metastasis Rev. 1999; 18:345-357), MHC class I (Wang, R., J. Mol. Med. 1999; 77:640-655), PTEN and phospho-inositol 3 kinase (PI3K) (12-14), the Ras oncogene product (Jiveskog, S., et al., J. Invest. Dermatol. 1998; 5:757-761), the stress kinases Jun amino-terminal kinase (JNK) and p38 (Itoh, S., et al., J Immunol. 1999; 162:7434-7440; Ivanov, V. N., et al., Oncogene 2000; 19:3003-3012), and their upstream regulator TRAF2 (Ivanov, V.N., et al., Oncogene 2000; 19:933-942), as well as signal-transducing molecules including β-catenin (Barker, N., et al., Adv Cancer Res. 2000; 77:1-24; Soldatenkov V A, et al., Cancer Res. 1999; 59:5085-8; Rubinfeld B, et al., Science 1997; 275:1790-2), and cell cycle regulators such as p16 (Piepkorn, M., J. Am. Acad. Dermatol. 2000; 42:705-726). Either mutation or altered expression has been reported for these regulatory proteins, which confer the changes implicated in the development and progression of human melanoma.
In comparing early- and late-stage melanoma cells we identified lower expression and activities of TRAF2 and its respective effectors germinal center kinase (GCK) and nuclear factor-kB (NF-kB) in early-stage melanoma cells (Ivanov, V. N., et al., Oncogene 2000; 19:933-942). Low expression levels of TRAF2/GCK in early-stage melanoma cells coincide with low level of c-Jun and NF-kB activities. Forced expression of GCK in these cells efficiently increased the resistance of the early-phase melanoma to radiation. Similarly, expression of the dominant negative form of GCK in late-stage melanoma reduced the resistance of late-phase melanoma cells lines to radiation (Ivanov, V. N., et al., Oncogene 2000; 19:933-942). These observations pointed to the changes in the regulation of key stress signaling molecules during melanoma progression and to the role of TRAF2 and its effector GCK in acquiring radiation resistance of late-phase human melanoma cells.
In elucidating transcription factors that may alter melanoma's resistance to UV-irradiation, we identified CREB associated proteins (Yang, Y. -M., et al., Oncogene 1996; 12:2223-2233) among which ATF2 was found to play an important role in acquiring such resistance (Ronai, Z., et al., Oncogene 1998; 16:523-531). Hypophosphorylated or transcriptionally inactive forms of ATF2 elicit a silencing effect on TNFα expression, which mediates an anti-apoptotic signal in LU1205, a late-stage melanoma cell line, resulting in increased apoptosis (Ivanov, V. N., et al., J. Biol. Chem. 1999; 274:14079-14089). The importance of the p38 signaling cascade, which is among major ATF2 kinases, in the biology of human melanoma was further demonstrated by the finding that p38 negatively regulates the expression of Fas via suppression of NF-kB transcriptional activity (Ivanov, V. N., et al., Oncogene 2000; 19:3003-3012). Thus, p38 appear to play a key role in acquiring the resistance of melanoma to radiation-induced apoptosis through its ATF2 effector, which upregulates TNFα expression, and via p38 direct suppression of NF-kB, which downregulates Fas expression.